Off-axis levelling in lithographic projection apparatus

ABSTRACT

In an off-axis levelling procedure a height map of the substrate is generated at a measurement station. The height map is referenced to a physical reference surface of the substrate table. The physical reference surface may be a surface in which is inset a transmission image sensor. At the exposure station the height of the physical reference surface is measured and related to the focal plane of the projection lens. The height map can then be used to determine the optimum height and/or tilt of substrate table to position the exposure area on the substrate in best focus during exposure. The same principles can be applied to (reflective) masks.

This is a continuation application of U.S. application Ser. No.09/519,875, filed Mar. 6, 2000, now U.S. Pat. No. 6,674,510, whichclaims priority from European Patent application No. 99200649.4, filedMar. 8, 1999, the entire contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention relates to height detection and levelling, forexample of the substrate and/or mask, in lithographic apparatus. Moreparticularly, the invention relates to a system for off-axis levellingin lithographic projection apparatus comprising:

-   -   a radiation system for supplying a projection beam of radiation;    -   a first object table provided with a mask holder for holding a        mask;    -   a second, movable object table provided with a substrate holder        for holding a substrate;    -   a projection system for imaging an irradiated portion of the        mask onto a target portion of the substrate; and    -   a positioning system for moving said second object table between        an exposure position, at which said projection system can image        said mask portion onto said substrate, and a measurement        position.

SUMMARY OF THE INVENTION

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection systems,including refractive optics, reflective optics, catadioptric systems,and charged particle optics, for example. The radiation system may alsoinclude elements operating according to any of these principles fordirecting, shaping or controlling the projection beam, and such elementsmay also be referred to below, collectively or singularly, as a “lens”.In addition, the first and second object tables may be referred to asthe “mask table” and the “substrate table”, respectively. Further, thelithographic apparatus may be of a type having two or more mask tablesand/or two or more substrate tables. In such “multiple stage” devicesthe additional tables may be used in parallel, or preparatory steps maybe carried out on one or more tables while one or more other tables arebeing used for exposures.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the mask(reticle) may contain a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto an exposure area(die) on a substrate (silicon wafer) which has been coated with a layerof photosensitive material (resist). In general, a single wafer willcontain a whole network of adjacent dies which are successivelyirradiated via the reticle, one at a time. In one type of lithographicprojection apparatus, each die is irradiated by exposing the entirereticle pattern onto the die at once; such an apparatus is commonlyreferred to as a wafer stepper. In an alternative apparatus—which iscommonly referred to as a step-and-scan apparatus—each die is irradiatedby progressively scanning the reticle pattern under the projection beamin a given reference direction (the “scanning” direction) whilesynchronously scanning the wafer table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally <1), the speed V at which the wafertable is scanned will be a factor M times that at which the reticletable is scanned. More information with regard to lithographic devicesas here described can be gleaned from International Patent ApplicationWO 97/33205, for example.

Until very recently, lithographic apparatus contained a single masktable and a single substrate table. However, machines are now becomingavailable in which there are at least two independently moveablesubstrate tables; see, for example, the multi-stage apparatus describedin International Patent Applications WO98/28665 and WO98/40791. Thebasic operating principle behind such multi-stage apparatus is that,while a first substrate table is at the exposure position underneath theprojection system for exposure of a first substrate located on thattable, a second substrate table can run to a loading position, dischargea previously exposed substrate, pick up a new substrate, perform someinitial measurements on the new substrate and then stand ready totransfer the new substrate to the exposure position underneath theprojection system as soon as exposure of the first substrate iscompleted; the cycle then repeats. In this manner it is possible toincrease substantially the machine throughput, which in turn improvesthe cost of ownership of the machine. It should be understood that thesame principle could be used with just one substrate table which ismoved between exposure and measurement positions.

The measurements performed on the substrate at the measurement positionmay, for example, include a determination of the spatial relationship(in X & Y directions) between various contemplated exposure areas on thesubstrate (“dies”), reference markers on the substrate and at least onereference marker (e.g. fiducial) located on the substrate table outsidethe area of the substrate. Such information can subsequently be employedat the exposure position to perform a fast and accurate X and Ypositioning of the exposure areas with respect to the projection beam;for more information see WO 99/32940 (P-0079), for example. Thisdocument also describes the preparation at the measurement position of aheight map relating the Z position of the substrate surface at variouspoints to a reference plane of the substrate holder. However thereference plane is defined by a Z-interferometer at the measurementposition and a different Z-interferometer is used at the exposureposition. It is therefore necessary to know accurately the relationshipbetween the origins of the two Z-interferometers.

An object of the present invention is to provide a system for off-axislevelling a substrate in a lithographic projection apparatus that avoidsthe need to relate the origins of two interferometer systems and enablesadditional improvements in positioning of the exposure areas duringexposure processes.

According to the present invention there is provided a lithographicprojection apparatus comprising:

-   -   a radiation system for supplying a projection beam of radiation;    -   a first object table provided with a mask holder for holding a        mask;    -   a second, movable object table provided with a substrate holder        for holding a substrate;    -   a projection system for imaging an irradiated portion of the        mask onto a target portion of the substrate; and    -   a positioning system for moving said second object table between        an exposure station, at which said projection system can image        said mask portion onto said substrate, and a measurement        station; characterized in that    -   said second object table has a physical reference surface fixed        thereto;    -   and by:    -   height mapping means located at said measurement station and        constructed and arranged to measure the height, relative to said        physical reference surface, of a plurality of points on the        surface of a substrate held on said substrate holder and to        create a height map thereof;    -   position measuring means located at said exposure station for        measuring the position of said physical reference surface in a        first direction substantially perpendicular to said substrate        surface, after movement of said second object table to said        exposure station; and    -   control means constructed and arranged to control the position        of said second object table in at least said first direction,        during exposure of said target portion, in accordance with said        height map and said position measured by said position measuring        means.

According to a further aspect of the present invention there is provideda method of manufacturing a device using a lithographic projectionapparatus comprising:

-   -   a radiation system for supplying a projection beam of radiation;    -   a first object table provided with a mask holder for holding a        mask;    -   a second, movable object table provided with a substrate holder        for holding a substrate; and    -   a projection system for imaging irradiated portions of the mask        onto target portions of the substrate at an exposure station;        the method comprising the steps of:    -   providing a mask bearing a pattern to said first object table;    -   providing a substrate having a radiation-sensitive layer to said        second object table; and    -   imaging said irradiated portions of the mask onto said target        portions of the substrate; characterized by the steps of:    -   before said step of imaging, generating, with the second object        table at a measurement station, a height map indicating the        height of a plurality of points on the substrate surface        relative to a physical reference surface on said second object        table;    -   moving the second object table to said exposure station and        measuring the position of said physical reference surface in a        first direction substantially perpendicular to said substrate        surface; and    -   during said step of imaging, positioning the second object table        in at least said first direction by reference to said height map        and said measured position in said first direction of said        physical reference surface.

In a manufacturing process using a lithographic projection apparatusaccording to the invention a pattern in a mask is imaged onto asubstrate which is at least partially covered by a layer ofenergy-sensitive material (resist). Prior to this imaging step, thesubstrate may undergo various procedures, such as priming, resistcoating and a soft bake. After exposure, the substrate may be subjectedto other procedures, such as a post-exposure bake (PEB), development, ahard bake and measurement/inspection of the imaged features. This arrayof procedures is used as a basis to pattern an individual layer of adevice, e.g. an IC. Such a patterned layer may then undergo variousprocesses such as etching, ion-implantation (doping), metallization,oxidation, chemo-mechanical polishing, etc., all intended to finish offan individual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array, of devices (dies) will be present on thesubstrate (wafer). These devices are then separated from one another bya technique such as dicing or sawing, whence the individual devices canbe mounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “exposurearea”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation or particle flux,including, but not limited to, ultraviolet radiation (e.g. at awavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm), extremeultraviolet radiation (EUV), X-rays, electrons and ions. Also herein,the invention is described using a reference system of orthogonal X, Yand Z directions and rotation about an axis parallel to the I directionis denoted Ri. Further, unless the context otherwise requires, the term“vertical” (Z) used herein is intended to refer to the direction normalto the substrate or mask surface, rather than implying any particularorientation of the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described below with reference toexemplary embodiments and the accompanying schematic drawings, in which:

FIG. 1 depicts a lithographic projection apparatus according to a firstembodiment of the invention;

FIG. 2 is a view showing how the wafer height is determined frommeasurements by the level sensor and the Z-interferometer;

FIGS. 3 to 6 are views showing various steps of the off-axis levellingprocedure according to the present invention;

FIG. 7 is a plan view of a substrate table showing the sensors andfiducials used in the off-axis levelling procedure according to thepresent invention;

FIG. 8 is a side view of the exposure and measurement stations in asecond embodiment of the invention;

FIG. 9 is a flow diagram illustrating various steps of the measurementprocess carried out at the measurement station of the second embodimentof the invention;

FIG. 10 is a flow diagram illustrating various steps of the exposureprocess carried out at the exposure station of the second embodiment ofthe present invention;

FIG. 11 is a diagram illustrating the scan pattern usable to measure theheight map of the present invention;

FIG. 12 is a diagram illustrating an alternative scan pattern usable tomeasure the height map of the present invention;

FIG. 13 is a diagram illustrating the global level contour process inthe second embodiment of the present invention;

FIG. 14 and its sub-FIGS. A to G illustrate the structure and operationof a presently preferred embodiment of a level sensor usable in theinvention;

FIG. 15 is a graph showing detector output vs. substrate table positionfor a capture spot of the level sensor of FIG. 14;

FIG. 15A is a diagram showing the arrangements of detector segments forthe capture spot of the level sensor of FIG. 14;

FIGS. 16 and 17 are diagrams illustrating a presently preferredembodiment of a confidence sensor usable in the second embodiment of theinvention;

FIG. 18 is a diagram of a beam splitter usable in the confidence sensorof FIGS. 16 and 17;

FIG. 19 is a diagram used to explain a Z-interferometer calibrationprocedure usable in embodiments of the invention;

FIG. 20 is a diagram illustrating the notation used in describing anexposure trajectory optimization procedure according to a thirdembodiment of the invention; and

FIG. 21 depicts a lithographic projection apparatus according to a fifthembodiment of the invention.

In the drawings, like references indicate like parts.

DETAILED DESCRIPTION OF VARIOUS EXEMPLARY EMBODIMENTS OF THE INVENTION

EMBODIMENT 1

FIG. 1 schematically depicts a lithographic projection apparatusaccording to the invention. The apparatus comprises:

-   -   a radiation system LA, Ex, IN, CO for supplying a projection        beam PB of radiation (e.g. UV or EUV radiation);    -   a first object table (mask table) MT provided with a mask holder        for holding a mask MA (e.g. a reticle), and connected to first        positioning means for accurately positioning the mask with        respect to item PL;    -   a second object table (substrate or wafer table) WTa provided        with a substrate holder for holding a substrate W (e.g. a        resist-coated silicon wafer), and connected to second        positioning means for accurately positioning the substrate with        respect to item PL;    -   a third object table (substrate or wafer table) WTb provided        with a substrate holder for holding a substrate W (e.g. a        resist-coated silicon wafer), and connected to third positioning        means for accurately positioning the substrate with respect to        item PL;    -   a measurement system MS for performing measurement        (characterization) processes on a substrate held on a substrate        table WTa or WTb at a measurement station;    -   a projection system (“lens”) PL (e.g. a refractive or        catadioptric system, a mirror group or an array of field        deflectors) for imaging an irradiated portion of the mask MA        onto an exposure area C (die) of a substrate W held in a        substrate table WTa or WTb at an exposure station.

As here depicted, the apparatus is of a transmissive type (i.e. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example.

The radiation system comprises a source LA (e.g. a Hg lamp, examinerlaser, an undulator provided around the path of an electron beam in astorage ring or synchrotron, a laser plasma source or an electron or ionbeam source) which produces a beam of radiation. This beam is passedalong various optical components comprised in the illuminationsystem,—e.g. beam shaping optics Ex, an integrator IN and a condenserCO—so that the resultant beam PB has a desired shape and intensitydistribution in its cross-section.

The beam PB subsequently intercepts the mask MA which is held in a maskholder on a mask table MT. Having passed through the mask MA, the beamPB passes through the lens PL, which focuses the beam PB onto anexposure area C of the substrate W. With the aid of the interferometricdisplacement and measuring means IF, the substrate tables WTa, WTb canbe moved accurately by the second and third positioning means, e.g. soas to position different exposure areas C, in the path of the beam PB.Similarly, the first positioning means can be used to accuratelyposition the mask MA with respect to the path of the beam PB, e.g. aftermechanical retrieval of the mask MA from a mask library. In general,movement of the object tables MT, WTa, WTb will be realized with the aidof a long stroke module (course positioning) and a short stroke module(fine positioning), which are not explicitly depicted in FIG. 1. In thecase of a waferstepper (as opposed to a step-and-scan apparatus) thereticle table may be connected only to a short stroke positioningdevice, to make fine adjustments in mask orientation and position.

The second and third positioning means may be constructed so as to beable to position their respective substrate tables WTa, WTb over a rangeencompassing both the exposure station under projection system PL andthe measurement station under the measurement system MS. Alternatively,the second and third positioning means may be replaced by separateexposure station and measurement station positioning systems forpositioning a substrate table in the respective exposure stations and atable exchange means for exchanging the substrate tables between the twopositioning systems. Suitable positioning systems are described, interalia, in WO 98/28665 and WO 98/40791 mentioned above. It should be notedthat a lithography apparatus may have multiple exposure stations and/ormultiple measurement stations and that the numbers of measurement andexposure stations may be different than each other and the total numberof stations need not equal the number of substrate tables. Indeed, theprinciple of separate exposure and measurement stations may be employedeven with a single substrate table.

The depicted apparatus can be used in two different modes:

1. In step-and-repeat (step) mode, the mask table MT is kept essentiallystationary, and an entire mask image is projected at once (i.e. a single“flash”) onto an exposure area C. The substrate table WT is then shiftedin the X and/or Y directions so that a different exposure area C can beirradiated by the beam PB;

2. In step-and-scan (scan) mode, essentially the same scenario applies,except that a given exposure area C is not exposed in a single “flash”.Instead, the mask table MT is movable in a given direction (theso-called “scan direction”, e.g. the Y direction) with a speed v, sothat the projection beam PB is caused to scan over a mask image;concurrently, the substrate table WTa or WTb is moved in the same oropposite direction at a speed V=Mv, in which M is the magnification ofthe lens PL (typically, M=¼ or ⅕). In this manner, a relatively largeexposure area C can be exposed, without having to compromise onresolution.

An important factor influencing the imaging quality of a lithographicapparatus is the accuracy with which the mask image is focused on thesubstrate. In practice, since the scope for adjusting the position ofthe focal plane of the projection system PL is limited and the depth offocus of that system is small, this means that the exposure area of thewafer (substrate) must be positioned precisely in the focal plane of theprojection system PL. To do this, it is of course necessary to know boththe position of the focal plane of the projection system PL and theposition of the top surface of the wafer. Wafers are polished to a veryhigh degree of flatness but nevertheless deviation of the wafer surfacefrom perfect flatness (referred to as “unflatness”) of sufficientmagnitude noticeably to affect focus accuracy can occur. Unflatness maybe caused, for example, by variations in wafer thickness, distortion ofthe shape of the wafer or contaminants on the wafer holder. The presenceof structures due to previous process steps also significantly affectsthe wafer height (flatness). In the present invention, the cause ofunflatness is largely irrelevant; only the height of the top surface ofthe wafer is considered. Unless the context otherwise requires,references below to “the wafer surface” refer to the top surface of thewafer onto which will be projected the mask image.

According to the invention, after loading a wafer onto the substratetable, the height of the wafer surface Z_(Wafer) relative to a physicalreference surface of the substrate table is mapped. This process iscarried out at the measurement station using a first sensor, referred toas the level sensor, which measures the vertical (Z) position of thephysical reference surface and the vertical position of the wafersurface, Z_(LS), at a plurality of points, and a second sensor, forexample a Z-interferometer, which simultaneously measures the verticalposition of the substrate table, Z_(IF) at the same points. As shown inFIG. 2, the wafer surface height is determined asZ_(Wafer)=Z_(LS)−Z_(IF). The substrate table carrying the wafer is thentransferred to the exposure station and the vertical position of thephysical reference surface is again determined. The height map is thenreferred to in positioning the wafer at the correct vertical positionduring the exposure process. This procedure is described in more detailbelow with reference to FIGS. 3 to 6.

As shown in FIG. 3, first the substrate table is moved so that aphysical reference surface fixed to the substrate table is underneaththe level sensor LS. The physical reference surface may be anyconvenient surface whose position in X, Y and Z on the substrate tablewill not change during processing of a wafer in the lithographicapparatus and, most importantly, in the transfer of the substrate tablebetween measurement and exposure stations. The physical referencesurface may be part of a fiducial containing other alignment markers andshould have such properties as allow its vertical position to bemeasured by the same sensor as measures the vertical position of thewafer surface. In a presently preferred embodiment the physicalreference surface is a reflective surface in a fiducial in which isinset a so-called transmission image sensor (TIS). The TIS is describedfurther below.

The level sensor may be, for example, an optical sensor such as thatdescribed in U.S. Pat. No. 5,191,200 (P-0039) (referred to therein as afocus error detection system); alternatively, a pneumatic or capacitivesensor (for example) is conceivable. A presently preferred form ofsensor making use of Moiré patterns formed between the image of aprojection grating reflected by the wafer surface and a fixed detectiongrating is described below in relation to a second embodiment of theinvention. The level sensor may measure the vertical position of aplurality of positions simultaneously and for each may measure theaverage height of a small area, so averaging out unflatnesses of highspatial frequencies.

Simultaneously with the measurement of the vertical position of aphysical reference surface by the level sensor LS, the vertical positionof the substrate table is measured using the Z-interferometer, Z_(IF).The Z-interferometer may, for example, be part of a three, five orsix-axis interferometric metrology system such as that described in WO99/28790 (P-0077) or WO 99/32940 (P-0079). The Z-interferometer systempreferably measures the vertical position of the substrate table at apoint having the same position in the XY plane as the calibratedmeasurement position of the level sensor LS. This may be done bymeasuring the vertical position of two opposite sides of the substratetable WT at points in line with the measurement position of the levelsensor and interpolating/modelling between them. This ensures that, inthe event that the wafer table is tilted out of the XY plane, theZ-interferometer measurement correctly indicates the vertical positionof the substrate table under the level sensor.

Preferably, this process is repeated with at least a second physicalreference surface spaced apart, e.g. diagonally, from the first physicalreference surface. Height measurements from two or more positions canthen be used to define a reference plane.

The simultaneous measurement of the vertical position of one or morephysical reference surfaces and the vertical position of the substratetable establishes a point or points determining the reference planerelative to which the wafer height is to be mapped. A Z-interferometerof the type mentioned above is effectively a displacement sensor ratherthan an absolute sensor, and so requires zeroing, but provides a highlylinear position measurement over a wide range. On the other hand,suitable level sensors, e.g. those mentioned above, may provide anabsolute position measurement with respect to an externally definedreference plane (i.e. nominal zero) but over a smaller range. Where suchsensors are used, it is convenient to move the substrate tablevertically under the level sensor until the physical referencesurface(s) is (are) positioned at a nominal zero in the middle of themeasurement range of the level sensor and to read out the currentinterferometer Z value. One or more of these measurements on physicalreference surfaces will establish the reference plane for the heightmapping. The Z-interferometer is then zeroed with reference to thereference plane. In this way the reference plane is related to thephysical surface on the substrate table and the Z_(Wafer) height map ismade independent of the initial zero position of the Z-interferometer atthe measurement station and other local factors such as any unflatnessin the base plate over which the substrate table is moved. Additionally,the height map is made independent of any drift in the zero position ofthe level sensor.

As illustrated in FIG. 4, once the reference plane has been established,the substrate table is moved so that the wafer surface is scannedunderneath the level sensor to make the height map. The verticalposition of the wafer surface and the vertical position of the substratetable are measured at a plurality of points of known XY position andsubtracted from each other to give the wafer height at the known XYpositions. These wafer height values form the wafer height map which canbe recorded in any suitable form. For example, the wafer height valuesand XY coordinates may be stored together in so-called indivisiblepairs. Alternatively, the points at which wafer height values are takenmay be predetermined, e.g. by scanning the wafer along a predeterminedpath at a predetermined speed and making measurements at predeterminedintervals, so that a simple list or array of height values (optionallytogether with a small number of parameters defining the measurementpattern and/or a starting point) may suffice to define the height map.

The motion of the substrate table during the height mapping scan islargely only in the XY plane. However, if the level sensor LS is of atype which only gives a reliable zero reading, the substrate table isalso moved vertically to keep the wafer surface at the zero position ofthe level sensor. The wafer height is then essentially derived from theZ movements of the substrate table, as measured by the Z-interferometer,necessary to maintain a zero readout from the level sensor. However, itis preferable to use a level sensor that has an appreciable measurementrange over which its output is linearly related to wafer height, or canbe linearized. Such measurement range ideally encompasses the maximumexpected, or permissible, variation in wafer height. With such a sensor,the need for vertical movement of the substrate table during the scan isreduced or eliminated and the scan can be completed faster, since thescan speed is then limited by the sensor response time rather than bythe ability of the short stroke substrate table to track the contour ofthe wafer in three dimensions. Also, a sensor with an appreciable linearrange can allow the heights at a plurality of positions (e.g. an arrayof spots) to be measured simultaneously.

Next, the wafer table is moved to the exposure station and, as shown inFIG. 5, the (physical) reference surface is positioned under theprojection lens so as to allow a measurement of its vertical positionrelative to the focal plane of the projection lens. In a preferredembodiment, this is achieved using one or more transmission imagesensors (described below) whose detector is physically connected to thereference surface used in the earlier measurements. The transmissionimage sensor(s) can determine the vertical focus position of theprojected image from the mask under the projection lens. Armed with thismeasurement, the reference plane can be related to the focal plane ofthe projection lens and a path for the substrate table inthree-dimensions which keeps the wafer surface in optimum focus can bedetermined. One method by which this can be done is to calculate Z, Rxand Ry setpoints for a series of points along the scan path. Thesetpoints are determined using a least squares method so as to minimizethe difference between the wafer map data and the focus plane of theexposure slit image. For ease of calculation, the relative motion of theexposure slit image and wafer can be expressed as the slit movingrelative to a static wafer. The least squares criterion can then beexpressed as, for each time t, finding the values Z(t), Rx(t) and Ry(t)which give a minimum value of: $\begin{matrix}{{{LSQ}(t)} = {\frac{1}{s}\frac{1}{w}{\int_{{- s}/2}^{s/2}{\int_{{- w}/2}^{w/2}\quad{\left\lbrack {{w\left( {x,y} \right)} - \left( {{Z(t)} + {x \cdot {{Ry}(t)}} - {y \cdot {{Rx}(t)}}} \right)} \right\rbrack^{2}{\mathbb{d}x}{\mathbb{d}y}}}}}} & (1)\end{matrix}$where w(x,y) is the wafer height map and the exposure slit image is arectangular plane of width s in the scanning direction and length Wperpendicular to the scanning direction with its position defined byz(t), Rx(t) and Ry(t). The setpoints and the wafer trajectory can beexpressed as functions of either Y (position in the scanning direction)or t (time) since these are related by Y=y₀+v.t, where y₀ is thestarting position and v is the scanning speed.

As mentioned above, the physical reference surface(s) is (are)preferably a surface in which a transmission image sensor (TIS) isinset. As shown in FIG. 7, two sensors TIS1 and TIS2 are mounted on afiducial plate mounted to the top surface of the substrate table (WT,WTa or WTb), at diagonally opposite positions outside the area coveredby the wafer W. The fiducial plate is made of a highly stable materialwith a very low coefficient of thermal expansion, e.g. Invar, and has aflat reflective upper surface which may carry markers used in alignmentprocesses. TIS1 and TIS2 are sensors used to determine directly thevertical (and horizontal) position of the aerial image of the projectionlens. They comprise apertures in the respective surface close behindwhich is placed a photodetector sensitive to the radiation used for theexposure process. To determine the position of the focal plane, theprojection lens projects into space an image of a TIS pattern TIS-Mprovided on the mask MA and having contrasting light and dark regions.The substrate stage is then scanned horizontally (in one or preferablytwo directions) and vertically so that the aperture of the TIS, passesthrough the space where the aerial image is expected to be. As the TISaperture passes through the light and dark portions of the image of theTIS pattern, the output of the photodetector will fluctuate. Thevertical level at which the rate of change of amplitude of thephotodetector output is highest indicates the level at which the imageof TIS pattern has the greatest contrast and hence indicates the planeof optimum focus. An example of a TIS of this type is described ingreater detail in U.S. Pat. No. 4,540,277. Instead of the TIS, aReflection Image Sensor (RIS) such as that described in U.S. Pat. No.5,144,363 may also be used.

Using the surface of the TIS as the physical reference surface has theadvantage that the TIS measurement directly relates the reference planeused for the height map to the focal plane of the projection lens and sothe height map can be employed directly to give height corrections forthe wafer stage during the exposure process. This is illustrated in FIG.6, which shows the substrate table WT as positioned under the control ofthe Z-interferometer at a height determined by the height map so thatthe wafer surface is at the correct position under the projection lensPL.

The TIS surface may additionally carry reference markers whose positionis detected using a through-the-lens (TTL) alignment system to align thesubstrate table to the mask. Such an alignment system is described inEP-0,467,445 A (P-0032), for example. Alignment of individual exposureareas can also be carried out, or may be obviated by an alignmentprocedure carried out at the measurement stage to align the exposureareas to the reference markers on the wafer stage. Such a procedure isdescribed in EP-0 906 590 A (P-0070) for example.

It will be appreciated that the mask image projected by the projectionsystem PL in a production process, in both step-and-repeat andstep-and-scan modes, is not a single point but extends over asignificant area in the XY plane. Since the wafer height may varysignificantly over this area it is desirable to optimize the focus overthe whole of the projection area, rather than just at a single point. Inembodiments of the present invention, this can be achieved bycontrolling not only the vertical position of the substrate table WT,but also its tilt about the X and Y axes (Rx, Ry). With knowledge of thelocation and extent of the intended exposure areas, the height mapgenerated by the present invention can be used to calculate in advanceoptimum Z, Rx and Ry position setpoints for the substrate table for eachexposure. This avoids the time required for levelling in known apparatusthat only measure wafer height when the wafer is positioned under theprojection lens and hence increases throughput. The optimum Z, Rx and Rysetpoints may be calculated by various known mathematical techniques,for example using an iterative process to minimize defocus (defined asthe distance between the wafer surface and the ideal focal plane), LSQ(t), integrated over the exposure area.

A further advantage is possible in the step-and-scan mode. In this mode,the projection lens projects an image of only part of the mask patternonto a corresponding part of the exposure area. The mask and substrateare then scanned in synchronism through the object and image focalplanes of the projection system PL so that the entire mask pattern isimaged onto the whole exposure area. Although in practice the projectionlens is held stationary and the mask and substrate are moved, it isoften convenient to consider this process in terms of an image slitmoving over the wafer surface. With the height map determined in advanceby the present invention, it is possible to calculate a sequence of Z,Rx and Ry setpoints matched to the XY scan path (usually, scanning takesplace in only one direction, e.g. Y). The sequence of setpoints can beoptimized according to additional criteria, e.g. to minimize verticalaccelerations or tilt motions that might reduce throughput or induceundesirable vibrations. Given a sequence of spaced-apart setpoints, ascanning trajectory for the exposure can be calculated using apolynomial or spline fitting procedure.

Whilst the present invention aims to position the wafer at the optimumposition in Z, Rx and Ry for a given exposure, the variations in wafersurface height over the exposure area may be such that the wafer cannotbe positioned to give adequate focus over the entire area. Suchso-called focus spots can result in an exposure failure. However, withthe present invention such failures can be predicted in advance andremedial action can be taken. For example, the wafer may be stripped andrecoated without the detrimental effect of further processing a badlyexposed wafer. Alternatively, if the predicted failure affects only oneor a few devices on the wafer while others will be acceptable,throughput may be enhanced by skipping exposures that can be predictedin advance to result in defective devices.

A further advantage of focus-spot detection can be derived from analysisof height maps taken. When large deviations from a global wafer planeare present in a wafer height map, this could indicate focus spots dueto substrate unflatness or process influences. Comparing wafer heightmaps from several wafers can indicate focus spots due to contaminationor unflatness of the substrate table. When focus spots appear atidentical or near-identical positions for different wafers, this is mostlikely caused by substrate holder contamination (so-called“chuck-spots”). From one wafer height map, one can also compare theheight map (topology) from repeated exposure areas (dies). If largedifferences occur at certain dies with respect to an average height map,one can suspect focus spots due to either wafer processing or thesubstrate table. Instead of comparing wafer height maps, the samecomparisons can also be done on the derived exposure paths per die, oron the defocus parameters MA, MSD or Moving Focus explained below. Whena certain die or wafer deviates much from an average exposure path ordefocus parameters, focus spots can also be detected.

All of the above mentioned analysis can be done before a wafer isexposed, and remedial action, such as wafer rejection (processinginfluences) or substrate holder cleaning (chuck spots), can be taken.With these methods, focus spots can be localised to the size of themeasurement spot of the level sensor 10. This implies a much higherresolution than previous methods of focus spot detection.

EMBODIMENT 2

A second embodiment of the present invention is shown in FIG. 8, whichshows only the exposure and measurement stations and only componentsrelevant to the discussion below. The second embodiment utilizes thelevelling principle of the present invention described above, togetherwith certain refinements described below.

At the exposure station, to the left of FIG. 8, the projection lens PLis shown mounted to metrology frame MF and projecting an image of TISmarker TIS-M on mask MA onto the sensor TIS mounted to wafer table WT.The metrology frame is isolated from the transmission of vibrations fromother parts of the apparatus and has mounted on it only passivecomponents used for fine metrology and alignment sensing. The wholemetrology frame may be made of a material with a very low coefficient ofthermal expansion, such as Invar, so that it provides a very stableplatform for the most sensitive measuring devices of the apparatus. Thecomponents mounted on the metrology frame MF include mirrors 34 and 35to which the measurement beams of the Z-interferometer Z_(IF) aredirected by 45°-mirrors 31 mounted on the sides of the wafer table WT.To ensure that the Z position of the substrate table can be measuredthroughout its range of movement in X, the mirrors 34, 35 have acorrespondingly large extent in the X direction. To ensure the Zposition can be measured throughout the range of Y-movement, the mirrors31 cover the whole length of the wafer table. Also mounted to themetrology frame MF are the beam generating and receiving parts 21 a, 22a of a confidence sensor 20 a described further below.

At the measurement station(on the right in FIG. 8), the same metrologyframe MF carries mirrors 33 and 32 which serve the same function as themirrors 34, 35 at the exposure station; again mirrors 32, 33 will have alarge extent in the X direction to accommodate the required range ofmovement of the substrate table WT which is just as large as that at theexposure station. Level sensor 10, comprising beam generating part 11and detection part 12, is also mounted on the metrology frame MF.Additionally, a confidence sensor 20 b, essentially the same asconfidence sensor 20 a at the exposure position, is provided. Othermeasurement devices, for example an alignment module, can also beprovided.

As discussed above, the use of the physical reference surface (again inthis embodiment this is provided by the upper surface of the TIS)relates the wafer height map to the wafer stage and makes it independentof the zero positions of the two Z-interferometers and certain localfactors such as unflatness of the base plate (stone) BP over which thewafer tables move. However, since the wafer height map is generatedusing the Z-interferometer at the measurement station and the substratetable position is controlled at the exposure station using a differentZ-interferometer provided there, any differences as a function of XYposition between the two Z-interferometers can affect the accuracy withwhich the wafer surface is positioned in the focal plane. The principalcause of these variations in an interferometer system of the type usedin the present invention is unflatness of the mirrors 32, 33, 34, 35.The 45° mirrors 31 are attached to the wafer table WT and travel with itas it swaps between exposure and measurement stations. Any unflatness ofthese mirrors therefore has largely the same effect on positioning atthe exposure station as at the measurement station, and largely cancelsout. However, the mirrors 32, 33, 34 and 35 mounted on the metrologyframe MF stay with their respective interferometers and so anydifferences in the surface contours of the corresponding pairs 32, 34and 33, 35 can adversely affect the vertical positioning accuracy of thesubstrate table WT.

The confidence sensors 20 a and 20 b are used at initial set-up of theapparatus, and periodically as required thereafter, to calibrate thedifferences between the Z-interferometers at the measurement andexposure stations. The confidence sensors are sensors capable ofmeasuring the vertical position of the upper surface of the wafer at oneor more points as the substrate table is scanned underneath it.Confidence sensors 20 a and 20 b can be similar in design to levelsensor 10 but need not be; since they are used only at setup (and forinfrequent recalibration) and with a reference wafer rather thanproduction wafers, the design criteria are less onerous and advantagecan be taken of this to design a simpler sensor. Conversely, theexistence of the projection lens PL at the exposure station willrestrict the physical locations available for the confidence sensor atthat station, and this also needs to be taken into account in design orselection of each confidence sensor. High accuracy is required of theconfidence sensors since the calibration they are used for will affectthe quality of every exposure.

In the calibration process using the confidence sensor(s), a referencewafer is loaded onto the substrate table. The reference wafer ispreferably a bare silicon wafer. There is no requirement for it to beany flatter than a normal bare Si wafer but its surface finish (in termsof reflectivity) is preferably optimized for the confidence sensors. Ina preferred embodiment of the invention the reference wafer ispreferably polished to maximize its reflectivity and minimizeunflatness.

In the calibration procedure, a partial height map of the referencewafer (as usual referenced to the physical reference surface) isgenerated at the measurement station using the confidence sensor 20 binstead of the level sensor 10. This is done in the same manner as withthe level sensor 10: the physical reference surface (TIS) is positionedat the zero point of the confidence sensor to zero the Z-interferometer,the wafer is then scanned under the confidence sensor, and the heightmap, is generated from the difference between the confidence sensor andZ-interferometer readings. A height map is also generated at theexposure station using the confidence sensor 20 a at the same points asthe measurement station height map. For this calibration, the heightmaps need not be a complete scan of the wafer; they need only coverstrips corresponding to the movement of the Z-interferometer beam overthe mirrors 32-35. (The order in which the maps are created is notimportant, provided the wafer is stable on the substrate table whileboth are done.)

Since the height maps represent the same wafer, any differences betweenthem will be caused by differences between the measurement systems usedto create them. The two confidence sensors are static, so their effectson the height maps will not be position-dependent and can be eliminatedby normalizing the two height maps and/or subtracting static offsets.Any remaining differences will be position-dependent, and the two heightmaps can be subtracted from one another to generate correction tables(mirror maps) that relates the exposure station Z-interferometer to themeasurement station Z-interferometer. These correction tables can beattributed to the differences between the mirrors 33, 35 and 32, 34attached to the metrology frame MF and can be applied to the waferheight maps generated in a production process, or used to correct one ofthe Z-interferometers used to generate the map or to position thesubstrate table during the exposure. Depending on the preciseconstruction of the Z-interferometers, particularly the metrology framemirrors and substrate table mirrors, the differences in Z positioncaused by the unflatnesses of the mirrors in each interferometer systemmay also be tilt dependent in one or more degrees of freedom (Rx, Ry,Rz). To eliminate this tilt dependence it may be necessary to use theconfidence sensors to create several height maps with the wafer stage atvarious different tilts, from which a number of different correctiontables (mirror maps) can be derived, as necessary.

Having described the principle of the off-axis levelling procedure, nowwill be described some further refinements to it that are employed inthe second embodiment, as well as how it is integrated into theproduction process. FIGS. 9 and 10 are referred to and respectively showthe steps carried out at the measurement station and at the exposurestation. In a lithography apparatus using two wafer tables, one tablewill be going through the steps of FIG. 9 while a second simultaneouslygoes through the steps of FIG. 10 before they are swapped. In thedescription below, the “life” of a single wafer is followed frommeasurement station (FIG. 9) to exposure station (FIG. 10) and back.

Starting at step S1 in FIG. 9, a wafer coated with a photosensitiveresist is loaded on to the substrate table WT. (Note that this maygenerally take place at a loading station separate from the measurementstation at which the substrate table is out of range of theinterferometer system IF.) The wafer table is moved into the capturerange(s) of one or more position sensitive devices (PSDs) so that aninitial coarse zeroing of the interferometric metrology system can beperformed, step S2. After the initial coarse zeroing, the fineinitialization/zeroing of the interferometric system follows in steps S3and S4. These two steps contain the level sensor measurements (denoted“LS”) on the (two or more) physical reference surfaces, which willdefine the reference plane (fixed to the wafer table) with respect towhich the wafer height map is measured. Also, two alignment measurements(denoted “AA”) are done on markers located on the same physicalreference surfaces, so as to define the horizontal reference positionsfixed to the wafer table. These measurements in S3 and S4 effectivelyzero the interferometric system in all degrees of freedom.

The next step in the levelling procedure is step S5, referred to as theglobal level contour (GLC). In this step, which is described furtherbelow, a wafer capture and an initial scan of the wafer with the levelsensor is made to determine its overall height and tilt as well as itsapproximate height at the points where the subsequent detailed scan willmove onto or off the wafer. This information enables the substrate tabletrajectory for the wafer height map scan to be defined.

In step S6, a global alignment of the wafer is done. At least twoalignment markers on the wafer are measured (W1 and W2), meaning thattheir XY position is determined with respect to the reference markers onthe TIS fiducials. This determines to what extent the wafer ishorizontally rotated (Rz) with respect to the scan direction (y), and isdone to be able to correct the wafer rotation such that the wafer heightmap scans are done parallel to the exposure area axis (i.e. “goingstraight over the exposure areas”).

After that, the levelling procedure continues with measurementsnecessary for a process dependent correction (PDC). A process dependentcorrection is necessary with some forms of level sensor, and will now beexplained.

The wafer height map must be taken each time a wafer is exposed. If awafer has already been subjected to one or more process steps, thesurface layer will no longer be pure polished silicon and there may alsobe structures or topology representing the features already created onthe wafer. Different surface layers and structures can affect the levelsensor readings and in particular can alter its linearity. If the levelsensor is optical, these effects may, for example, be due to diffractioneffects caused by the surface structure or by wavelength dependence inthe surface reflectivity, and cannot always be predicted. To determinethe required process dependent correction, an exposure area or die isscanned under the level sensor with the substrate table WT set toseveral different vertical positions spanning the linear or linearizedrange of the level sensor 10. The wafer height, i.e. the physicaldistance between the wafer surface and the reference plane, should notchange with the vertical position of the substrate table; it is obtainedby subtracting the measurements of the level sensor andZ-interferometer: Z_(WAFER)=Z_(LS)−Z_(IF). Therefore if the determinedvalue of Z_(WAFER) does change with vertical position of the substratetable this indicates that either or both the level sensor orZ-interferometer are not linear or not equally scaled. TheZ-interferometer is deemed to be linear since it looks at the mirrors onthe wafer table and metrology frame; and in fact is linear to a greaterextent than the required accuracy for the wafer height map, at leastonce the correction determined by the use of the confidence sensor isapplied. Therefore, any differences in the wafer height values areassumed to result from non-linearity or mis-scaling of the level sensor.They, and the knowledge of at which level sensor readings they wereobserved, can be used to correct the output of the level sensor. It hasbeen found in a presently preferred embodiment of the level sensor thata simple gain correction is sufficient, but a more complex correctionmay be required for other sensors.

If the wafer to be processed has exposure areas on it that have beensubjected to different processes, then a process-dependent correction isdetermined for each different type of exposure area on the wafer.Conversely, if a batch of wafers having exposure areas that haveundergone the same or similar processes are to be exposed, it may onlybe necessary to measure the process-dependent correction for each typeof exposure area once per batch. That correction can then be appliedeach time that type of exposure area is height-mapped in the batch.

In many IC fabs, the photosensitive resist is applied to the waferimmediately before it is loaded into the lithography apparatus. Forthis, and other, reasons, the wafer may be at a different temperaturethan the substrate table when it is loaded and clamped in place. Whenthe wafer cools (or warms) to the same temperature as the substratetable, thermal stresses can be set up because the wafer is clamped veryrigidly using vacuum suction. These may result in undesirable distortionof the wafer. Thermal equilibrium is likely to have been reached by thetime the steps S2 to S7 have been completed. Therefore, at step S8, thevacuum clamping the wafer to the substrate table is released, to allowthe thermal stresses in the wafer to relax, and then reapplied. Thisrelaxation may cause small shifts in the position and/or tilt of thewafer but these are acceptable since steps S2 to S4 are independent ofthe wafer and S5 and S6 are only coarse measurements. Any shift in thewafer position at this stage does not affect the process-dependentcorrection since that is a calibration of the level sensor rather than ameasurement of the wafer.

After the vacuum has been reapplied, and from here on it is not releasedagain until the exposure process is finished, the Z-map is carried outat step S9. The scan required for the Z-map must measure the height ofsufficient points to enable the wafer to be positioned during exposureat the desired accuracy. It is also important that the points measuredcover the actual area where the wafer is to be exposed; measurementstaken over non-exposure areas, such as scribe lanes and so-called mousebites, may give misleading results. Accordingly, the height mapping scanmust be optimized to the specific pattern of exposure areas on the waferat hand; this is described further below.

Once the Z-map is completed, the advance alignment measurements, stepS10, are carried out before the substrate table is swapped, at step S11,to the exposure position. In the advance alignment process, thepositions of a number of alignment markers on the wafer relative to thereference markers F located on the TIS fiducial (physical referencesurface) fixed to the substrate table are accurately determined. Thisprocess is not particularly relevant to the present invention and so isnot described further herein.

In the swap procedure, the substrate table carrying the height-mappedwafer arrives at the exposure station, step S13 in FIG. 10. A coarseposition determination of the substrate table is made at step S14 and,if necessary, a new mask MA is loaded onto the mask table MT, step S15.The mask loading process may be carried out, or at least begun,simultaneously with the substrate table swap. Once a mask is in positionand a coarse position determination, step S14, has been made, a firstTIS scan is carried out using sensor TIS1 at step S16. The TIS scanmeasures the vertical and horizontal position of the substrate table atwhich the TIS is located in the aerial image focus of the projectionlens, as described above, yielding a focal plane reference. Since theheight map generated as step S9 in FIG. 9 is referenced to the physicalsurface in which the TIS is located, the vertical positions of thesubstrate table necessary to put the wafer surface in the focal planefor the different exposure areas are directly derived. A second TISscan, step S17, is also carried out using sensor TIS2, yielding a secondpoint for referencing a focal plane.

Once the TIS scans have been completed and the focal plane determined,the exposure process S18 is carried out, optionally after any necessarysystem calibrations in step S19 (e.g. adjustments to correct for lensheating effects). The exposure process will generally involve theexposure of multiple exposure areas using one or more masks. Wheremultiple masks are used, after mask exchange S20, one TIS scan S17 canbe repeated to update any focal plane changes. Between some or allexposures, the system calibration step S19 may also be repeated. Aftercompletion of all exposures, the substrate table carrying the exposedwafer is swapped at step S13 for the substrate table carrying the waferthat has meanwhile been undergoing steps S1 to S10 of FIG. 9. Thesubstrate table carrying the exposed wafer is moved to the loadingstation and the exposed wafer taken out so that a fresh wafer can beloaded and the cycle can resume.

To explain the wafer height mapping scan of step S9 of FIG. 9, FIG. 11,shows an example of a pattern of exposure areas C of various shapes andsizes arranged on a wafer to make best use of the silicon area. Thedifferent exposure areas C are separated by scribe lanes SL andgenerally-triangular unused areas, known as “mouse-bites” are inevitablyleft between the rectangular exposure areas and the curved edge of thewafer. The scribe lanes are where the wafer will be cut once allproduction processes have been completed (so as to separate thedifferent devices) and some cutting techniques may require that thescribe lanes in one direction all span the entire width of the wafer; inthat case it is convenient to orient these full wafer-width scribe lanesparallel to the scanning direction (e.g. the Y direction) if theapparatus is to be used in step-and-scan mode. The scribe lanes andmouse bites may not be exposed, and so after the wafer has beensubjected to a few process steps or depositions of layers they may havevery different heights and surface properties than the exposure areas C.Accordingly it is important to disregard any height measurements inthese areas, which are not going to be exposed.

A presently preferred embodiment of the level sensor uses a linear arrayof, e.g., nine optical spots arranged perpendicular to the scanningdirection to measure the height at nine points (areas) simultaneously.(Note that the Z-interferometer data can also be interpolated to providecorresponding Z-position data of the substrate table at an array ofcorresponding level sensor points.) The array of spots is of a sizesufficient to cover the width of the widest exposure area that can beexposed in the apparatus.

The presently preferred scanning scheme is to scan the array of spots ina meander path 50 such that the center spot of the array passes alongthe midline of each column of exposure areas; this midline correspondsto the midline of the illuminated slit in the exposure process. The datathus generated can be directly related to the exposure scan with aminimum of rearrangement or calculation. This method also eliminatespart of the mirror unflatness effect, since, at both measurement andexposure stations, scans are carried out with the Z-interferometer beampointing at the same position on the mirrors 31 attached to thesubstrate table. If the column of dies is narrower than the array ofspots of the level sensor, data obtained from the spots not lying whollywithin the exposure area are ignored. In other embodiments of the levelsensor it may be possible to adjust the width of the array of spots tomatch the width of the exposure areas.

If a wafer has some exposure areas whose center lines are offset in thedirection perpendicular to the scanning direction from those of theremainder, a modified scanning scheme may be used to advantage. Thissituation is illustrated in FIG. 12 which shows one row of dies E whosecenter lines are offset from the remaining dies D. In such a case, themap can be created more quickly and with fewer accelerations for thesubstrate table by scanning two meander paths. One path, referenced 52in FIG. 12, covers one set of exposure areas D and the other, referenced53, covers the others E. Of course, other arrangements of the exposureareas may require further modifications to the scanning scheme.

Where the level sensor has a limited linear or linearized range, whichis likely the case, the substrate table WT must be scanned underneath itat a vertical position that brings the wafer surface into that range.Once the wafer surface has been found it is a simple matter, by means ofa closed feedback loop of the level sensor reading to the substratetable positioning system, to adjust the vertical position of thesubstrate table WT to keep the wafer surface in the linear or linearizedrange but it is not so simple to find the wafer surface when the levelsensor first moves onto an exposure area from outside the wafer. In ameander path there are several such in-points, referenced 51 andindicated by arrows on the meander path 50 in FIG. 11, compounding theproblem.

To find the wafer surface at the in-points 51 it is possible to providea capture spot in advance of the main level sensor spot array. Thereflection of the capture spot on the wafer is then directed to adetector that has a wider capture range than is the case for the mainspots. This, however, requires additional hardware: a capture spot onboth sides of the main spots (before/after) or a restriction to scanningin only one direction. An alternative, not necessarily requiringadditional hardware, is to stop the substrate table close to eachin-point, perform a wafer capture and measure the wafer surface in thelinear or linearized range of the level sensor to approximate the wafersurface position at the in-point. This however slows down themeasurement procedure significantly, which may have undesirableconsequences for throughput.

In this embodiment of the invention, these problems are avoided byperforming a global level contour scan mentioned above (step S5 in FIG.9) after the wafer surface is captured. The global level contour scan isexplained further with reference to FIG. 13.

For the global level contour scan the substrate table is firstpositioned so that a convenient point (preferably near the edge) withinan exposure area C is underneath a single capture spot and the mainspots of the level sensor (spot array). The wafer surface is found, e.g.by scanning the substrate table vertically until the wafer surface iscaptured and comes within the linear or linearized range of the mainspots, and then the substrate table is scanned so that the central spot41 traverses a path 60 around the inside of the perimeter of the totalexposure area. The capture procedure is described further below.Measurements of the wafer surface height are taken at defined positionsaround the scan. Where other spots of the array as well as the centerspot fall over (exposure areas of) the wafer, the measurements fromthese spots, as well as the central one, can also be taken. However,measurements should not be taken from spots falling outside the exposureareas. As illustrated, the global level contour path 60 is a windingpath following the edges of the exposure areas quite closely; however asmoother path may also be employed and, particularly when the wafer iswell filled with exposure areas, a circular course 61 may well sufficeand be more convenient. The global level contour may also be arranged asa circle passing over mouse bites, in which case measurements are nottaken over the mouse bites, or the data of any measurements taken onmouse bites are disregarded in calculation of the global height and tiltof the wafer.

The data gathered in the global level contour scan are used for twopurposes. Firstly data relating to the wafer height in the vicinity ofthe in-points 51 (see FIG. 11) of the height mapping scan to be carriedout later are used to predict the wafer height at the in-points 51 sothat the substrate table can be brought to the correct height to get thewafer surface position in the linear or linearized level sensor rangeduring the mapping scan. In most cases only a few data points arerequired for this and they need not be particularly close to thein-points to allow a sufficiently accurate prediction of the waferheight to be determined by interpolation or extrapolation. It is alsodesirable to know the local Ry tilt at the in-points 51 for the heightmapping scan, since the level sensor has an array of spots in theX-direction which (preferably) all need to be brought within theirlinear or linearized ranges. If the global level contour scan isparallel, or nearly parallel, to the Y direction in the vicinity of anyin-point, the Ry tilt cannot be accurately determined using dataobtained from only a single spot. Where a level sensor having an arrayof measurement spots spaced apart in the X direction, such as thatdescribed below, is used, data from multiple spots can be used todetermine a local Ry tilt. Of course, data from spots lying within theexposure area are selected if part of the array falls outside that area.

The second use of the global level contour data is to determine aglobal, or average, height and tilt (around 2 axes) for the whole wafer.This is done by known mathematical techniques, e.g. a least-squaresmethod, to determine a plane that most closely fits the wafer heightdata gathered. If the global tilt (sometimes referred to as the “wedge”)is greater than a predetermined amount, this may well indicate anincorrect loading procedure. In that case the wafer can be unloaded andreloaded for a retry and even rejected if it continues to fail. Theglobal height and tilt information is used to focus an advance alignmentsensor used in step S10 of FIG. 9 to accurately determine the spatialrelationship of alignment markers on the wafer to reference markers onthe substrate stage. The advance alignment sensor and process aredescribed in greater detail in WO 98/39689 (P-0070).

During a wafermap scan, the level sensor 10 provides continuous Z and Ryfeedback signals to the substrate table to keep the level sensor 10 inits linear or linearized range. If this feedback loop stops (the levelsensor 10 doesn't supply correct numbers) the table is controlled byfollowing a path corresponding to the global wafer wedge (a Z profileaccording to global Rx).

A presently preferred embodiment of the level sensor 10 is illustratedin FIG. 14 and will be explained below additionally with reference toFIGS. 14A to 14G, which show aspects of the operation of the sensor.

Level sensor 10 comprises a beam generation branch 11 which directs ameasurement beam b_(LS) onto the wafer W (or the physical referenceplane when the vertical position of that is being measured, or anyreflecting surface) and a detection branch 12 which measures theposition of the reflected beam, which is dependent on the verticalposition of the wafer surface.

In the beam generation branch, the measurement beam is generated bylight source 111, which may be an array of light emitting or laserdiodes, or generated elsewhere and passed to “illuminator” 111 byoptical fibers. The beam emitted by light source 111 preferably containsa wide band of wavelengths, e.g. from about 600 to 1050 nm, so as toaverage out any wavelength dependence of interference effects from thewafer surface, particularly after some process steps have beencompleted. Illumination optics 112, which may include any suitablecombination of lenses and mirrors, collect the light emitted by lightsource 111 and evenly illuminate projection grating 113. Projectiongrating 113 is shown in greater detail in FIG. 14A and consists of anelongate grating 113 a, which may be divided to generate an array ofseparate/discrete spots, with grating lines parallel to its axis, and anadditional aperture 113 b which forms a capture spot ahead of the maindetection spot array on the wafer. The period of the grating will bedetermined in part by the accuracy at which the wafer surface positionis to be measured and may, for example be about 30 μm. The projectiongrating is positioned with a small rotation around its optical axis suchthat the grating lines projected on the wafer are not parallel to anywafer coordinate axis, thereby to avoid interference with structures onthe wafer which are along the x or y direction. Projection lens 114 is atelecentric system which projects an image of the projection grating 113onto the wafer W. Projection lens 114 preferably consists essentially oronly of reflecting optical elements so as to minimize or avoid chromaticaberration in the projected image; since the projection beam isbroadband these cannot easily be eliminated or compensated for in arefractive optical system. Folding mirrors 115, 116 are used to bringthe projection beam b_(LS) into and out of the projection lens 114 andpermit a convenient arrangement of the components of the beam generationbranch.

The projection beam b_(LS) is incident on the wafer at a fairly largeangle, α, to the normal, e.g. in the range of from 60° to 80°, and isreflected into the detection branch 12. As shown in FIG. 14B, if thewafer surface WS shifts in position by a distance Δh to position WS′,then the reflected beam r′ will be shifted relative to the beam r, priorto the shift in the wafer surface, by a distance 2.Δh.sin(α). FIG. 14Balso shows the appearance of the image on the wafer surface; because ofthe large angle of incidence, the image is spread out perpendicular tothe grating lines.

The reflected beam is collected by detection optics 121 and focused ondetection grating 126, which is essentially a copy of projection grating113 and is sub-divided to correspond to the spot-array pattern.Detection optics 121 are directly complementary to projection optics 114and will also consist essentially or only of reflective elements, tominimize chromatic aberration. Again folding mirrors 122, 123 may beused to enable a convenient arrangement of the components. Betweendetection optics 121 and the detection grating 126 are positioned alinear polarizer 124 to polarize the light at 45° and a birefringentcrystal 125 which causes a shear perpendicular to the grating linesequal in magnitude to the grating period between the horizontal andvertical polarized components of the light. FIG. 14C shows the beam asit would be at the detection grating 126 without the birefringentcrystal; it is a series of alternating light and dark bands with thelight bands polarized at 45°. The birefringent crystal 125 shifts thehorizontal and vertical polarization states so that the light bands ofthe horizontal polarization component fill the dark bands of thevertical polarization component. As shown in FIG. 14D, the illuminationat the detection grating 126 is therefore uniform grey but has stripesof alternating polarization state. FIG. 14E shows the detection grating126 overlaid on this pattern, which depends on the vertical position ofthe wafer surface; when the wafer is at a nominal zero verticalposition, the detection grating 126 will overly and block half of thelight bands of one polarization state, e.g. the vertical, and half ofthe other state.

The light passed by the detection grating 126 is collected by modulationoptics 127 and focused on detector 128. Modulation optics include anpolarization modulation device driven by an alternating signal, e.g.with a frequency of about 50 kHz, so as to pass the two polarizationstates alternately. The image seen by the detector 128 thereforealternates between the two states shown in FIG. 14F. Detector 128 isdivided into a number of regions corresponding to the array of spotswhose height is to be measured. The output of a region of detector 128is shown in FIG. 14G. It is an alternating signal with period equal tothat of the modulating optics and the amplitude of the oscillationsindicates the degree of alignment of the reflected image of theprojection grating on the detection grating, and hence the verticalposition of the wafer surface. As mentioned above, if the wafer surfaceis at the nominal zero position, the detection grating 126 will blockout half of the vertical polarization state and half of the horizontalpolarization state so that the measured intensities are equal and theamplitude of the oscillating signals output by the detector regions willbe zero. As the vertical position of the wafer surface moves away fromthe zero position, the detection grating 126 will begin to pass more ofthe horizontally polarized bands and block more of the verticallypolarized bands. The amplitude of the oscillations will then increase.The amplitude of the oscillations, which is a measure of the verticalposition of the wafer surface, is not directly linearly related to thevertical position of the wafer surface in nanometers. However, acorrection table or formula can readily be determined on initial setupof the apparatus (and periodically recalibrated if necessary) bymeasuring the constant height of the surface of a bare silicon wafer atvarious different vertical positions of the substrate table, using thecalibrated Z-interferometer and uncalibrated level sensor 10.

To ensure that the measurements of the level sensor and theZ-interferometer are taken simultaneously, a synchronization bus isprovided. The synchronization bus carries clock signals of a very stablefrequency generated by a master clock of the apparatus. Both the levelsensor and Z-interferometer are connected to the synchronization bus anduse the clock signals from the bus to, determine sampling points oftheir detectors.

The capture spot 113 b passed by the projection grating 113 passes thedetection grating, where it is incident on three separate detectionregions, two 131, 133 set high and one 132 set low, as shown in FIG.15A. The output from the low detection region is subtracted from thoseof the high regions. The capture spot detector regions are arranged sothat when the wafer surface is at the zero position, the capture spotfalls equally on the high and low detection regions and the subtractedoutput is zero. Away from the zero position, more of the capture spotwill fall on one of the detection regions than the other and thesubtracted output will increase in magnitude with its sign indicatingwhether the wafer is too high or too low. The dependence of thesubtracted detector output d_(cap) on substrate table position Z_(IF) isillustrated in FIG. 15. This form of detector output allows a fasterzero capture method than a conventional servo feedback. According to theimproved method, referred to as “move-until”, when the capture spotdetector indicates that the wafer surface is too high or too low, theZ-position actuators of the substrate table are instructed to move thestage in the appropriate direction to bring the wafer surface into thelinear or linearized range of the main level sensor array. The movementof the wafer stage continues until the output of the capture spotdetector d_(cap) passes a trigger level t_(h) or t_(l) according towhich direction it is traveling. Crossing the trigger level causes theapparatus control to issue a command to the Z-position actuators tobegin a braking procedure. The trigger levels are set so that, in theresponse time and the time taken to brake the stage motion, the stagewill move to, or close to, the zero position. Thereafter the stage canbe brought to the zero position under control of the more accurate mainlevel sensor spots. The trigger points will be determined in accordancewith the dynamics of the stage and need not be symmetrically spacedabout zero detector output. This “move-until” control strategy enables arapid and robust zero capture without requiring a linear measurementsystem, and can be used in other situations.

The level sensor described above can be further optimized to improve itsperformance. Improvement in accuracy in the scan (Y) direction can beeffected by appropriate signal filtering and this may be adapted tospecific process layers observed on partly processed wafers. Additionalimprovements (for specific process layers) in all directions may beobtained by optimization of the measurement spot geometry, which can beadjusted by changing the illumination optics 112 (to adjust theuniformity and/or angular distribution of the illumination light on theprojection grating 113), by changing the projection grating 113 or byadjusting the detection system (size, position and/or angular resolutionof the detector and the number of detectors).

A presently preferred form of the confidence sensors 20 a, 20 b isillustrated in FIGS. 16 and 17. The beam generation branch 21 comprisesa light source 211 (e.g. a solid state laser diode or super-luminescentdiode) which emits light of limited bandwidth. It is convenientlysituated away from the metrology frame and its output brought to thedesired point by an optical fiber 212. The light is output from fiberterminator 213 and directed onto a beam splitter 215 by collimatingoptics 214. Beam splitter 215 creates two parallel measurement beamsb_(cs1) and b_(cs2) which are focused to evenly illuminate respectivespots 23 on the wafer W by telecentric projection optics 216. Since themeasurement beams of the confidence sensor have a limited bandwidth,projection optics 216 can conveniently employ refractive elements.Detection optics 221 collect the reflected beams and focus them at theedge of detection prism 222 which is positioned between detectors 223,224 and detection optics 221. As shown in FIG. 17, which is a side viewof detection prism 222 and detector 223, a measurement beam is incidenton the back of detection prism 222 and exits through angled faces 222 a,222 b. Detector 223 consists of two detector elements 223 a, 223 bpositioned so that light emerging from face 222 a of detection prism 222reaches detector element 223 a and that emerging from face 222 b reachesdetector element 223 b. Detector 224 is similar. Outputs of detectorelements 223 a and 223 b are intensity-scaled and subtracted. When thewafer surface is at the zero position, the measurement beam fallssymmetrically on faces 222 a, 222 b of detection prism 222 and equalamounts of light will be directed to detector elements 223 a and 223 b.These will then give equal outputs and so the subtracted output will bezero. As the wafer surface moves away from the zero position, theposition of the reflected beam will move up or down and fall more on oneof faces 222 a, 222 b than on the other resulting in more light beingdirected to the respective detector element so that the subtractedoutput will change proportionally. A tilt of the wafer can be determinedby comparison of the outputs of detectors 223 and 224.

This arrangement provides a simple and robust height and level detectorthat can be used as the confidence sensor in the second embodiment ofthe present invention as well as in other applications. The confidencesensor is primarily intended for initial set up and periodic, e.g.monthly, recalibration of the Z-interferometers of the measurement andexposure stations. However, the confidence sensor described above has awider capture zone and more rapid response than the TIS used for precisedetermination of the position of the focal plane of the projection lensPL relative to substrate table WT. Accordingly, the confidence sensor 20a can advantageously be used, when the substrate table is first swappedto the exposure station, to make a coarse determination of the verticalposition of the TIS. The height measured by the confidence sensor isrelated to previously measured best focus position(s) and used topredict a starting point and range for the TIS scan near the expectedposition of the best focal plane. This means that the TIS scan,described above, can be made shorter and hence quicker, improvingthroughput.

A beam splitter 215 that can be used in the confidence sensors is shownin FIG. 18. A beam splitter is composed of a number of prisms which aremade from the same glass and are preferably of equal thickness. Thebasic operation principle is described using a beam splitter consistingof 3 prisms 51, 52, 53. The first prism 51 is trapezoidal incross-section and the input beam 54 is incident normally on its top face55 near one side. The position of the input beam 54 is such that itmeets one side face 56 of the first prism 51 which is at 45° to the topface 55. The second prism 52 is joined onto the side face 56 of firstprism 51 and the join is coated so that a desired proportion of theinput beam (half in the present embodiment) continues into the secondprism 52 to form beam 57 while the remainder is reflected horizontallywithin the first prism 51 to form beam 58. Beam 58 reflected in thefirst prism 51 meets the second side face 59 of that prism, which isparallel to the first side face 56 and is reflected downwards out of thelower face of the first prism 51 and through top and bottom faces ofthird prism 53 which are parallel to top face 55 of first prism 51. Thesecond side face 59 may be coated as necessary to ensure total internalreflection of beam 58. The beam 57, which passed into the second prism52, is reflected internally by two parallel faces of the second prism52, which are perpendicular to the side face 56 of the first prism 51,and emerges from the bottom face of the second prism 52 which isparallel to the top face 55 of first prism 51. Beams 57 and 58 arethereby output in parallel, but are offset. The separation between thebeams 57, 58 is determined by the sizes of the prisms 51 and 52. Prism53 is provided to equalize the optical path lengths of beams 57, 58 sothat the imaging optics for both beams can be identical. Prism 53 alsosupports prism 52 as illustrated but this may not be necessary in someapplications. To enhance the reflection of the beam 57 at the surfacewhere prisms 52 and 53 meet, a void may be left or a suitable coatingprovided.

The beam splitter 50 is simple, robust and easy to construct. Itprovides output beams in parallel (whereas a conventional cubic beamsplitter provides perpendicular beams) and with equal path length. Thesplitting surface can be made polarization selective or not, and in thelatter case can divide the input beam intensity evenly or unevenly asdesired.

It is a feature of the level and confidence sensors described above, aswell as other optical height sensors, that they are insensitive to tiltof the wafer stage about an axis perpendicular to the Z-directiondefined by the intersection of the wafer surface WS and the focus planeof the measurement spot of the level sensor 10. This is due to the factthat the sensors measure a height over the area of the measurement spotextrapolated to the spot's focus axis. The tilt insensitivity can beused to calibrate the Z-interferometers and the optical sensors towardseach other in the XY plane. The procedure for such calibration isdescribed with reference to FIG. 19 and the level sensor, but a similarprocedure can be used with the confidence sensor or any other similaroptical sensor.

The positioning system of the substrate table is linked to themulti-axis interferometer system of which the Z-interferometer is apart, and can be set to apply a rotation about a selected axis in the XYplane using spaced-apart Z-actuators. To align the Z-interferometermeasurement position with the level sensor measurement spot, thepositioning system is used to rotate the stage about an axis passingthrough the Z-interferometer measurement position and parallel to, forexample, the Y axis. The Z position of the table as measured by theZ-interferometer will remain unchanged during this tilt. If the levelsensor and Z-interferometer are exactly aligned, then the wafer surfaceposition will also remain unchanged. However, if the level sensormeasurement position is offset from the Z-interferometer position by anamount δX, as shown in FIG. 19, then tilting the substrate table WT tothe position shown in phantom in that Figure will cause a change δW_(LS)in the level sensor output. The offset δX, and the offset δY in the Ydirection, can therefore be quickly determined by detecting any changein level sensor output with tilts about two, preferably perpendicular,axes passing through the Z-interferometer position. The parameters ofthe interferometer system or the level sensor 10 can then be adjusted toensure that the Z-interferometer measurement position is exactlyopposite the level sensor measurement position.

Where the level sensor uses an array of measurement spots, it cannotalways be ensured that the spots are exactly aligned. The abovetechnique can therefore be used to determine any offsets of theindividual spots from the nominal position with respect to theZ-interferometer position. This information can then be used to correctthe height map or the level sensor output.

EMBODIMENT 3

The third embodiment employs the levelling principle of the firstembodiment and is the same as that embodiment except as described below.The third embodiment may also make use of the hardware and refinementsof the second embodiment, described above. However, the third embodimentmakes use of an improved method for optimization of the exposure path.This is explained below with reference to FIG. 20.

As discussed above, it is convenient and valid to consider that thesubstrate stage is stationary and that the exposure slit image moves,even though in practice it is the wafer that moves. The explanationbelow is given from this view point.

FIG. 20 illustrates the notations used below. It should be noted that,although the slit image SI is depicted for clarity in FIG. 20 spacedfrom the wafer surface, the aim of the optimization procedure is toensure that during an exposure the focus plane of the slit imagecoincides as far as possible to the wafer surface. Considering a onedimensional wafer whose surface is defined by w(y) and a slit image SI,the moving average (over time) defocus MA(y) corresponding to acoordinate on the wafer can be calculated from: $\begin{matrix}{{{MA}(y)} = {\frac{1}{s}{\int_{{- s}/2}^{s/2}{\left\lbrack {{w(y)} - \left\lbrack {{z\left( {y + v} \right)} - {v \cdot {{Rx}\left( {y + v} \right)}}} \right\rbrack} \right\rbrack \cdot {\mathbb{d}v}}}}} & (2)\end{matrix}$where the integral is taken over the slit size, s, in the scan directionand the integrand w(y)−[z(y+v)−v.Rx(y+v)] is the focus error on a pointof the wafer at a certain moment in time. Similarly, the moving standarddeviation for a point on the wafer can be defined as: $\begin{matrix}{{{MSD}^{2}(y)} = {\frac{1}{s}{\int_{{- s}/2}^{s/2}{\left\lbrack {{w(y)} - \left\lbrack {{z\left( {y + v} \right)} - {{v \cdot {Rx}}\left( {y + v} \right)}} \right\rbrack - {{MA}(y)}} \right\rbrack^{2}{\mathbb{d}v}}}}} & (3)\end{matrix}$which is the defocus variation in time during the actual exposure ofthat point on the wafer. To minimize the difference between the plane ofthe exposure slit image and the wafer, a quadratic defocus term is used,defined as follows: $\begin{matrix}{\left. {{M\quad{F^{\quad 2}(y)}} = {\frac{1}{s}{\int_{{- s}/2}^{s/2}\left\lbrack {{w(y)} - \left\lbrack {{z\left( {y + v} \right)} - {v \cdot {{Rx}\left( {y + v} \right)}}} \right\rbrack - {{MA}(y)}} \right\rbrack}}} \right\rbrack^{2}{\mathbb{d}v}} & (4)\end{matrix}$where MF(y) is called the moving focus. It will be seen that MF(y) canalso be written in terms of MA(y) and MSD(y) as follows:MF ²(y)=MA ²(y)+MSD ²(y)  (5)

This means that in the optimization of the exposure path andminimisation of the moving focus over the exposure area, both the movingaverage and the moving standard deviation are taken into account, incontrast to the simpler least-squares optimization of the firstembodiment, which neglects any time, and thus scanning, integration.Equations [3] and [4] can easily be extended to two dimensions by addingRy(t) dependency and integrating MF over X from −W/2 to +W/2, where W isthe width of the slit in the X-direction. To calculate the optimizationit is convenient to use a frequency domain representation. Calculationin the frequency domain also enables high-frequency variations in thesetpoints, that would result in excessive substrate stage accelerationsin any or all of the degrees of freedom, to be filtered out, such thatthe exposure path is optimized for the performance of the substratetable positioning system.

In the above discussion, the optimum focus of the exposure slit image isassumed to conform to a plane; however, this is not necessarily thecase: the optimum focus may in fact lie on an arbitrary surface,resulting in a so-called focal plane deviation (FPD). If the contour ofthat surface over the exposure slit area can be measured using the TISto create a focus map f(x,y), or calculated, then the resulting data orequations can be added to the equations above so that the wafer motionis optimized for the actual optimum focal surface.

The optimization technique of the third embodiment can result in betterfocus for scanning systems and smoother substrate stage trajectories,increasing throughput and yield.

EMBODIMENT 4

In a fourth embodiment, the level sensor is provided with additionalfeatures to counteract errors in the measurement of the wafer surfaceposition that may be caused by interference between the beam reflectedby the top surface of the resist layer and the beam refracted into theresist layer and reflected by its bottom surface. Otherwise, the fourthembodiment may be the same as any of the first to third embodimentsdescribed above.

The interference of beams reflected from said top and bottom surface islargely dependent on the resist properties and wafer surface properties,as well as on the optical wavelength and angle of incidence of themeasurement beam. Broadband light sources and detectors are currentlyused to average out such single-wavelength interference effects.Improvement of this averaging principle can be realized if the wafersurface position is measured in a spectrally resolved manner, whereby adistinct measurement is performed for a number of wavelengths in thebroadband measurement beam. To achieve this, it is necessary to make atemporally or spatially separated wavelength (color) system formeasuring the wafer surface position. This necessitates changes such asthe following to the level sensor's measurement principle.

A first possible change to the level sensor is to replace the continuousbroadband light source by one capable of selectively generating lightbeams of different wavelength ranges (colors). This can, for example, beachieved by selectively interposing different color filters (e.g. on acarousel) at a suitable point in the level sensor's illumination system,by the use of several independently selectable light sources, by using awavelength-tunable light source, or by using a selected beam portionfrom a rotating/vibrating prism located in a small broadband beam. Thelevel sensor is then used to take several measurements of the wafersurface at each point, using different wavelengths of light in themeasurement beam.

Another option is to replace the broadband detector by one capable ofselectively detecting light of different wavelength ranges (colors).This can be achieved, for example, by placement of color filters in thedetection optics before the detector, by spatially splitting themeasurement beam for different wavelengths using a prism and thendetecting the different-wavelength beams on separate detectors, or byany other way of spectrally analyzing the broadband-reflected beam tomeasure the wafer surface position.

Naturally, it is also possible to use a combined approach, whereby boththe projection system and the detection system are adapted to achievespectral resolution.

In the absence of interference effects, each measurement (for eachwavelength) should give the same result; consequently, if differentresults are obtained in such measurements, this indicates the presenceof effects as referred to in the first paragraph above. An improvedwafer surface position measurement can then be derived using a varietyof techniques. For example, discrepant results may be corrected ordiscarded. Majority voting techniques may also be used. Alternatively,on the basis of a spectral measurement of the wafer surface position,one might even derive real positions by means of a model describing thespectral response of the resist and the wafer surface properties.

Since the described interference effect also depends on the angle ofincidence of the measurement beam on the wafer surface, one might alsowant to vary this angle of incidence so as to evaluate the effect andthen correct it. Accordingly, a further possible change to the levelsensor is to adapt it such that the wafer surface position can be madeusing measurement beams at different angles of incidence. One way toachieve this is to define multiple measurement beams having differentangles of incidence for the same spot on the wafer, but separateprojection and detection optics systems. Alternatively, one can changethe optical system so that the same projection and detection systemsencompass the different optical axes pertaining to the variousmeasurement beams. Another option, which generates temporally varyingangles of incidence, is to use rotating/translating folding mirrors (orother movable components) in the optical systems of the level sensor.

As with the wavelength dependence described above, in the absence ofinterference effects, measurements at different angles of incidenceshould give the same result. Therefore, any discrepancies (variationwith angle of incidence) can be avoided, compensated for, or modeled inthe same way.

The above-mentioned additional features and improvements may, of course,be used together or separately, and in other optical sensors than thosedescribed here.

EMBODIMENT 5

A fifth embodiment of the invention is shown in FIG. 21. The fifthembodiment of the invention is a lithography apparatus employing, as theexposure radiation, extreme ultraviolet (EUV) radiation, e.g. ofwavelength in the range of 9 to 16 nm, and a reflective mask MA′.Functionally at least, the components of the fifth embodiment aregenerally the same as those of the first embodiment but they are adaptedto the exposure radiation wavelength used and their arrangement isadjusted to accommodate the beam path necessitated by the use of areflective mask. Particular adaptions that may be necessary includeoptimizing the illumination and projection optics IL′, PL′ to thewavelength of the exposure radiation; this will generally involve theuse of reflective rather than refractive optical elements. An example ofan illumination optical system IL′ for use with EUV radiation isdescribed in European Patent Application 00300784.6 (P-0129).

An important difference between lithography apparatus using reflectivemasks and those using transmissive masks, is that with the reflectivemask, unflatness of the mask results in position errors on the waferthat are multiplied by the optical path length of the downstream opticalsystem, i.e. the projection lens PL′. This is because height and/or tiltdeviations of the mask locally change the effective angle of incidenceof the illumination beam on the mask and hence change the XY position ofthe image features on the wafer.

According to the fifth embodiment of the invention, the effects ofunflatness of the mask are avoided or alleviated by making a height mapof the mask in advance of the exposure and controlling the mask positionin at least one of Z, Rx and Ry during the exposure. The height map canbe generated in a similar manner to that described above (i.e. off-axislevelling of the mask at a measurement station); however, it may also begenerated with the mask at the exposure station, which may obviate theneed to relate the height map to a physical reference surface. Thecalculation of the optimum position(s) of the mask during the exposureor exposure scan (the exposure path) can be equivalent to that describedabove, but it may also be a coupled optimization of wafer and maskexposure paths. However, for a mask, it may be advantageous to placegreater weight in the optimization calculations on tilt deviations,since these will have a greater effect on the position at the wafer.

It should be explicitly noted that a lithographic projection apparatusaccording to the current invention may contain two (or more) substratetables and/or two (or more) mask tables. In such a scenario, it ispossible for a first substrate on a first substrate table to beundergoing height-mapping at the measurement station while a secondsubstrate on a second substrate table is concurrently undergoingexposure at the exposure station; and similarly in the case of multiplemask tables. Such a construction can greatly increase throughput.

It should also be explicitly noted that the current invention can beapplied to substrate leveling alone, to mask leveling alone, or to acombination of substrate leveling and mask leveling.

While we have described above specific embodiments of the invention itwill be appreciated that the invention may be practiced otherwise thanas described. The description is not intended to limit the invention.

1. A method of calibrating a lithographic projection apparatuscomprising: generating a first height map of a substrate by measuring aposition in a first direction substantially perpendicular to a surfaceof said substrate of a plurality of points on said substrate surface andsimultaneously measuring a position of a substrate holder using a firstposition detection system, at a measurement station of a lithographicsystem; generating a second height map of said substrate by measuring aposition in said first direction of said plurality of points on saidsubstrate surface and simultaneously measuring a position of saidsubstrate holder using a second position detection system, at anexposure station of the lithographic system; and comparing said firstand second height maps to calibrate said first and second positiondetection systems.
 2. A method according to claim 1, wherein thesubstrate is a bare silicon wafer.
 3. A method according to claim 2,wherein said bare silicon wafer has a surface that is polished.
 4. Amethod according to claim 1, wherein the first and second height maps ofsaid substrate are generated only for strips of the substrate surfacewhich correspond to the measurement ranges of the first and secondposition detection systems.
 5. A method according to claim 1, whereincomparing the first and second height maps comprises determining thedifference between the first and second height maps, and storingresulting correction values.
 6. A method according to claim 5, whereinthe correction values are applied to substrate height maps generatedduring subsequent production processes.
 7. A method according to claim5, wherein the correction values are used to correct the positiondetection systems.
 8. A method according to claim 1, wherein the firstand second height maps are normalised.
 9. A method according to claim 1,further comprising: determining static offsets between the first andsecond height maps; and removing said static offsets.
 10. A methodaccording to claim 1, further comprising: repeating a plurality of timessaid generating a first height map of said substrate, said generating asecond height map, and said comparing said first and second height maps,wherein a different tilt is applied to said substrate holder for eachrepetition.